The invention relates to a process for the manufacture and magnetisation of a magnetic powder containing bitumen foil, with which the magnetic powder is magnetised in a magnetisation station by a strong magnetic field.
A process of the aforementioned type is described in DE 37 11 810 A1. The magnetic powder, which is an isotropic material such as barium ferrite powder, is used in such bitumen foils so as to provisionally fix the damping foils in position during the installation until they are finally bonded to the backing during the passing through a furnace by the melting and re-hardening of an adhesive. The publication mentions that the ferrite particles, after manufacture of the foil, are converted into directional permanent magnets. This takes place at room temperature.
In practice it was found that often the magnetic forces of such bitumen foils are not sufficient or that a relatively large quantity of magnetic powder is required in order to achieve adequate magnetic forces. However, especially for reasons of weight it is desirable that the quantity of magnetic powder is as little as possible, since such bitumen foils are very frequently used in motor vehicles, where an increase in weight leads to an increase in fuel consumption and reduces the acceleration power.
The object of the invention is to develop a process of the aforementioned type, which leads to bitumen foils that at identical magnetic forces have a significantly lower weight per unit area than the bitumen foils manufactured according to the known process.
According to the invention this object is achieved in that prior to the magnetisation the bitumen foil is heated to such a temperature that the magnetic powder particles can align themselves by the effect of the magnetic field and in that immediately after the magnetisation the bitumen foil is cooled sufficiently in order to maintain the magnetisation.
By the heating of the bitumen foil according to the invention prior to the magnetisation, the magnetic powder particles are able to orient themselves in a preferred direction, which is preserved by the subsequent cooling. As a result considerably less magnetic powder is required to achieve an identical magnetic force as with bitumen foils according to the state of the art. With the bitumen foil according to the aforementioned DE 37 11 810 A1, for example, for its use in motor vehicles a magnetic powder portion of 60-70% by mass is required, whereas for a bitumen foil according to the process of the invention 30-40% by mass suffice. On the whole, the invention results in a weight saving of approximately 20% at an identical magnetic force and while retaining or even increasing the acoustic properties.
The process can be carried out in a particularly effective and economic manner when prior to the magnetisation the bitumen foil is heated to a temperature of over 100° C. and thereafter is cooled to at least approximately 70° C. By this cooling to at least approximately 70° C., the magnetisation and orientation is “frozen in”. Furthermore, at higher temperatures, when pulling the bitumen foil off a roller that is cooling it, a disorientation of the individual grains of the magnetic powder would occur and as a result thereof the achieved magnetisation would be reduced.
The energy requirement for the implementation of the process according to the invention is particularly low when according to a further development of the invention only a layer of the bitumen foil of approximately 1 mm thickness is heated to above 100° C.
The process can be carried out in a particularly simple manner when the magnetisation and cooling take place on a water-cooled magnetic roller that comprises permanent magnets.
Particularly strong magnetic forces can be obtained with a relatively small quantity of magnetic powder when as magnetic powder an anisotropic material is used.
It is advantageous when as magnetic powder anisotropic barium or strontium is used.
A sticking of the bituminous compound and the foil produced therefrom to a steel band of a steel band plant can be excluded when the bituminous compound for the manufacture of the bitumen foil is rolled by means of calibrating rollers onto a carrier foil that covers the steel band.
The carrier foil preferably is a polyester foil.
The process according to the invention can be carried out in various ways. For the further clarification of its basic principle, reference is made in the following to the drawing.